Process for obtaining viscous mineral oil from an underground deposit

ABSTRACT

The invention provides a process for producing mineral oil from an underground mineral oil deposit into which at least one injection well which is in contact with the mineral oil deposit via at least one perforation zone and at least one production well have been sunk, comprising injecting at least one aqueous urea solution into the injection well and pressure-injecting the aqueous urea solution through the perforation zone into the mineral oil deposit, wherein the aqueous urea solution on pressure injection into the mineral oil deposit is at a temperature of at least 80° C.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser. No. 61/499,710 filed Jun. 22, 2011, the entire contents of which are incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a process for producing mineral oil, especially viscous mineral oil, from mineral oil deposits, wherein the mineral oil yield is increased by injecting aqueous urea solution having a temperature of at least 80° C. into the deposit. The present process forms part of the group of mineral oil production technologies which are typically characterized as water-gas-steam flooding.

BACKGROUND

In natural mineral oil deposits, mineral oil occurs in cavities of porous reservoir rocks which are closed off from the surface of the earth by impervious overlying strata. In addition to mineral oil, including proportions of natural gas, a deposit further comprises water with a greater or lesser salt content. The cavities may be very fine cavities, capillaries, pores or the like, for example those having a diameter of only approx. 1 μm; the formation may additionally also have regions with pores of greater diameter and/or natural fractures.

After the well has been sunk into the oil-bearing strata, the oil at first flows to the production wells due to the natural deposit pressure, and erupts from the surface of the earth. This phase of mineral oil production is referred to by the person skilled in the art as primary production. In the case of poor deposit conditions, for example a high oil viscosity, rapidly declining deposit pressure or high flow resistances in the oil-bearing strata, eruptive production rapidly ceases. With primary production, it is possible on average to extract only 2 to 10% of the oil originally present in the deposit. In the case of higher-viscosity mineral oils, eruptive production is generally completely impossible.

In order to enhance the yield, what are called secondary production processes are therefore used.

The most commonly used process for secondary mineral oil production is water flooding. This involves injecting water into the oil-bearing strata through what are called injection wells. This artificially increases the deposit pressure and forces the oil from the injection wells out of the production wells. However, water flooding cannot substantially increase the yield level of viscous crude oils. In the ideal case of water flooding, a water front proceeding from the injection well should force the oil homogeneously over the entire mineral oil formation to the production well. In practice, a mineral oil formation, however, has regions with different levels of flow resistance. In addition to oil-saturated reservoir rocks which are of fine porosity and have a high flow resistance for water, there also exist regions with low flow resistance for water, for example natural or synthetic fractures or very permeable regions in the reservoir rock. Such permeable regions may also be regions from which oil has already been extracted. In the course of water flooding, the flooding water injected naturally flows principally through flow paths with low flow resistance from the injection well to the production well. The results of this are that the oil-saturated deposit regions which are of fine porosity and have high flow resistance are not flooded, and that increasingly more water and less mineral oil is produced via the production well. In this context, the person skilled in the art refers to “watering out of production”. The effects mentioned are particularly marked in the case of heavy or viscous mineral oils. The higher the mineral oil viscosity, the more likely it is that production will water out rapidly.

In order to reduce the oil viscosity and increase the oil extraction level, different processes based on steam flooding, CO₂ flooding, gas-water flooding and aqueous solutions of chemicals have been developed. These processes exhibit a distinctly improved yield as compared with conventional water flooding. Steam flooding involves injecting steam into the deposit to heat the mineral oil and thus reduce the oil viscosity. As in the case of water flooding, however, it is also possible for steam and steam condensate to penetrate undesirably rapidly through zones of high permeability from the injection wells to the production wells, and hence reduce the efficiency of tertiary production.

It is additionally known that aqueous solutions of chemicals which generate gases under particular conditions can be used for flooding. For example, CO₂ dissolves in the mineral oil and reduces the viscosity thereof. For example, RU 2007 113 251 A discloses a process in which an aqueous urea solution and steam are pressure-injected cyclically into the deposit. After the hydrolysis of the urea in the deposit, gases which bring about an increase in oil extraction are generated.

Bocksermann A., Kotscheschkov A. and Tarasov A. (“Vervollkommnung der thermischen Methoden zur Entölungssteigerung der Erdöllagerstatten” [Enhancement of the thermal methods for increasing oil extraction from mineral oil deposits], Russian Institute for Scientific and Technical Information; “Development of oil and gas deposits” series; volume 24, Moscow 1993) describe a process for deposit development in which water flooding/steam flooding and cyclic pumping of aqueous urea solution are conducted. Under the action of the deposit temperature and the steam temperature, the hydrolysis of the urea commences to form ammonia and carbon dioxide, which promote and enhance oil extraction. Disadvantages of this process are that it can be used only in deposits with temperatures of at least 80° C., and homogeneous hydrolysis of the urea in the deposit is difficult to achieve since the urea solution can also flow within cooled/cooler zones of the deposit. The hydrolysis of the urea in the deposit can be controlled only with difficulty, if at all.

Another problem for the use of aqueous urea solution is that the temperature range within a deposit which has already been developed for a few years is usually inhomogeneous. Exact prognoses for the temperature ranges are often very difficult or impossible, and so the prediction of the effect of the aqueous urea solution in the mineral oil deposit can be predicted only with difficulty, if at all.

A difficulty which can arise in the case of prolonged water flooding is the cooling of the zone close to the injector. Typically, a particular amount of aqueous urea solution is injected, followed by flooding with water, in order to displace the urea solution to sites in the deposit which have the elevated temperatures required for the urea hydrolysis. If the cooled zone around the zone close to the injector is relatively large, the urea solution has to be transported over a relatively great distance to reach deposit zones with the appropriate temperatures. In the course of this, the urea concentration of the injected urea solution is reduced significantly as a result of the subsequent flooding.

For irregular deposits, it is barely possible to predict the change in concentration of the aqueous urea solution underground, the exact definition of the temperature range of the deposit, and the paths that the flooding media will take. Therefore, the use of aqueous urea solution by the processes known is also always associated with the risk that a large portion of the aqueous urea solution injected ends up in deposit sections in which the urea hydrolysis is impossible.

As described above, the marked thermal hydrolysis of aqueous urea solutions begins only at temperatures of 80° C. Below 80° C., the thermal hydrolysis of the urea is too slow to form ammonia and carbon dioxide to a sufficient extent within an economically rationable timespan.

In order to allow urea to be used additionally in those mineral oil deposits whose temperature is below 80° C., proposals have been made in the prior art to heat the underground formations. U.S. Pat. No. 4,982,789 proposes for this purpose injecting hot water or steam into the deposit until the deposit has a temperature which is sufficient for the thermal hydrolysis of urea. This process has the disadvantage that the heating of a deposit entails enormous energy costs. Furthermore, the disadvantage exists that the heating of the deposit may likewise take place irregularly, and so, with this method as well, precise forecasts of the temperature fields and predictions of the effect of the subsequently injected aqueous urea solution in the mineral oil deposit are extremely difficult and complicated.

Processes in which water and gas are introduced alternately into the deposit have the disadvantage that the mixing of the gas phase with the liquid phase is difficult to achieve underground in the mineral oil deposit.

Described in the prior art, furthermore, are processes using urea solutions which even prior to injection into the borehole have temperatures which ensure thermal hydrolysis of the urea.

U.S. Pat. No. 5,209,295 discloses a process for lowering the viscosity of high-viscosity mineral oil in situ in a mineral oil deposit by injecting a mixture of steam and an aqueous urea solution. This mixture of steam and the aqueous urea solution may in this case be prepared above ground or at the borehole head.

U.S. Pat. No. 4,227,575 and U.S. Pat. No. 4,594,609 disclose processes for treating mineral oil deposits, again injecting steam, admixed with urea, into mineral oil deposits.

According to the teaching of U.S. Pat. No. 4,227,575, the mineral oil deposit is heated by the injection to of steam to temperatures of at least 260° C. The principle objective here is to modify the matrix of the mineral oil deposit, which comprises montmorillonite clays.

The advantage of these processes is that the thermal hydrolysis of the urea is ensured. A disadvantage, however, is that the decomposition of the urea begins as early as above ground or at the borehole head to a considerable extent, meaning that considerable amounts of carbon dioxide and ammonia are formed as early as above ground. Ammonia is a toxic and caustic gas, whose handling above ground necessitates special safety precautions, thereby giving rise to extra costs in plant construction. Carbon dioxide, especially in conjunction with steam, is extremely corrosive, and hence attacks the steel components of the mineral oil production plant. Particularly affected by this are the steel pipe carriers of the well, such as riser pipes or injection pipes, for example.

BRIEF SUMMARY

It was therefore an object of the invention to provide a process for producing mineral oil from mineral oil formations, which is also usable in deposits with low temperatures and which combines the advantages of gas flooding, gas-water flooding, steam flooding and optionally surfactant treatment. More particularly, the process is to be suitable for production of viscous mineral oil. A further object of the present invention was to provide a process by means of which the formation of poisonous ammonia and of corrosive carbon dioxide above ground or at the borehole head is substantially avoided.

This object is achieved by the following process for producing mineral oil from an underground mineral oil deposit into which at least one injection well which is in contact with the mineral oil deposit via at least one perforation zone and at least one production well have been sunk, comprising injecting at least one aqueous urea solution into the injection well and pressure-injecting the aqueous urea solution through the perforation zone into the mineral oil deposit, wherein the aqueous urea solution on pressure injection into the mineral oil deposit is at a temperature of at least 80° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical injection well as usable in the process according to the invention.

FIGS. 2-3 show two injection wells which can be used for preferred embodiments of the present process.

FIGS. 4 a and 4 b show the temperature profile of the aqueous urea solution along the injection well from the borehole head to the perforation zone (4).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment, this object is achieved by the following process for producing mineral oil from an underground mineral oil deposit into which at least one injection well which is in contact with the mineral oil deposit via at least one perforation zone and at least one production well have been sunk, comprising injecting at least one aqueous urea solution into the injection well and pressure-injecting the aqueous urea solution through the perforation zone into the mineral oil deposit, wherein the aqueous urea solution on pressure injection into the mineral oil deposit is at a temperature of at least 80° C., wherein the at least one aqueous urea solution is heated to at least 80° C. in the injection well by means of electrical and/or inductive heating elements arranged above the perforation zone in the injection well.

At temperatures of about 80° C., noticeable thermal hydrolysis of the aqueous urea solution commences. This means that the thermal hydrolysis of the urea to ammonia and carbon dioxide proceeds sufficiently rapidly that a sufficient amount of ammonia and carbon dioxide is formed within an economically viable period of time to bring about the desired increase in mineral oil production, for example within one to three days. If the profile is to be modified more quickly, the temperature that the aqueous urea solution has on pressure injection may be more than 80° C., for example at least 85° C. or at least 90° C.

Some of the gases which evolve in the course of hydrolysis of the aqueous urea solution are dissolved in the water (preferably ammonia) and form an alkali bank, some of the other gases formed (predominantly CO₂) dissolve partially in the oil, and the rest of the gases formed fill the liquid present with gas. The gas-filled flooding liquid has an increased viscosity and promotes profile modification. The aqueous bank enriched with gases and alkali is pressure-injected through the deposit. The formation of ammonium hydroxide reduces the interfacial tension between oil and water. Since the urea solution is what is called “a true solution” with a homogeneous urea concentration, it is simultaneously enriched with the gases formed in the hydrolysis. In the case of appropriate selection of the urea concentration in the aqueous solution, it is thus possible to generate water-gas mixtures in situ within the deposit.

The process according to the invention thus has the advantage that the hydrolysis of the urea is conducted under control, and the deposit is flooded with a mixture of water, ammonia and carbon dioxide. This avoids the use of costly technologies for the preparation and pumping of gas-water mixtures, which are typically produced above ground. The process according to the invention can be used in exploitation of deposits with different temperatures, different mineralizations of the formation water and different storage properties.

The following specific details can be given for the invention:

The process according to the invention for production of mineral oil is a process for secondary or tertiary mineral oil production, which means that it is employed after primary mineral oil production due to the intrinsic pressure of the deposit has ceased and the pressure in the deposit has to be maintained by injecting flooding media. The flooding medium, especially water and/or steam, is injected into the deposit through at least one injection well, and crude oil is withdrawn from the deposit through at least one production well. The term “crude oil” in this context of course does not mean single-phase oil; instead what is meant is the customary emulsions which comprise oil and formation water and are produced from mineral oil deposits.

Deposits:

The deposits may be deposits for all kinds of oil, for example those for light or heavy oil. The process is preferably used for production of viscous mineral oil having a viscosity of at least 30 cP, preferably of at least 50 cP, measured at deposit temperature. Preferably in accordance with the invention, the deposits are heavy oil deposits, which means deposits comprising mineral oil with an API gravity of less than 22.3° API. Mineral oil is generally produced from such deposits by injecting steam.

The initial deposit temperature—which means the temperature before employment of the process according to the invention—is typically within the range from 8° C. to 120° C., preferably 8° C. to 100° C., more preferably 8° C. to 90° C., even more preferably 8° C. to 80° C. and especially 8° C. to 70° C.

The deposit temperature may, however, be altered by prior flooding with flooding media when the temperature of the flooding medium differs from the temperature of the deposit. For example, the temperature of an initially cooler deposit can increase in the course of prolonged flooding with steam; in the case of prolonged flooding with water, the temperature of an initially warmer deposit can fall. In this case, the temperature change takes place essentially in the region close to the injection well and in the region between the injection and production wells. The expression “region between the injection and production wells” relates to those volume elements of the deposit through which the flow paths lead from the injection wells to the production wells, though the flow paths of course need not necessarily proceed in a straight line between injection and production wells. The person skilled in the art is aware of methods by which such volume elements through which flow proceeds can be determined. The reference points for determination of the “region between the injection and production wells” are of course not the production well and the injection well over their entire length, but rather, in the case of the injection well, the point in the well at which the formulation (F) actually enters the formation from the injection well, and, in the case of the production well, that point at which crude oil actually enters the production well from the formation, or is to enter it in the future.

Preferably in accordance with the invention, the present process is used when the temperature close to the injection well and in the region between the injection and production wells is 8° C. to 70° C., preferably 8° C. to 40° C. and more preferably 8° C. to 20° C.

Process

To execute the process, at least one production well and at least one injection well are sunk into the mineral oil deposits. In general, a deposit is provided with several injection wells and with several production wells. These may be wells which have already been used at an earlier stage of mineral oil production, for example in the course of a prior water flooding or steam flooding operation, but the wells may also have been sunk into the deposit specially to execute the process according to the invention.

The process according to the invention comprises the injecting of at least one aqueous urea solution into the at least one injection well and pressure-injecting of the at least one aqueous urea solution through the at least one perforation zone into the mineral oil deposit, wherein the at least one aqueous urea solution on pressure injection into the mineral oil deposit is at a temperature of at least 80° C., preferably of at least 85° C. and more preferably of at least 90° C. The aqueous urea solution is in this case heated to at least 80° C. in the injection well by means of electrical and/or inductive heating elements arranged above the perforation zone in the injection well.

Urea is converted in the presence of water in the course of hydrolysis to ammonia and carbon dioxide according to the following equation:

H₂N—CO—NH₂+H₂O→2NH₃+CO₂.

The carbon dioxide formed dissolves partially in the mineral oil and reduces the viscosity thereof. The ammonia formed dissolves in the water present in the deposit and forms an alkaline ammonia buffer system with a pH of 9 to 10. This buffer system has a surfactant-like effect in the deposit. As a result of the production of gas in the water and the formation of alkali, foam-like structures which cause an increase in viscosity of the water used for flooding are also formed. This serves to modify the profile of the flooding and to balance out the flood front. If surface-active substances are additionally used with the alkaline bank, they may bring about a further increase in the level of oil extraction. Surface-active substances and surface-active components are understood in the present case as materials which can reduce the surface tension of water. Preference is given here to surfactants such as the nonionic, anionic and cationic surfactants listed below.

From about 80° C., the thermal hydrolysis of the urea solution proceeds sufficiently rapidly to bring about an increase in mineral oil production within economically viable periods. Significant amounts of urea are converted above the melting temperature of the urea (133° C.). In the case of an increase in the temperature to 200° C., the urea is decomposed to an extent of 75% within a relatively short time, and, in the case of very rapid heating to 280° C., the reaction proceeds almost to completion. Above 400° C., urea is converted completely to gaseous components, which means that no solid residues remain.

The aqueous urea solutions are typically provided above ground by dissolving the urea in water. It is optionally possible to add further additives, such as surface-active components (surfactants) and ammonium salts.

Typically, the aqueous urea solution comprises 3 to 79% by weight of urea and 21 to 97% by weight of water, based on the total weight of the urea solution. The aqueous urea solution preferably comprises 20 to 40% by weight of urea and 80% by weight to 60% by weight of water, based on the total weight of the urea solution.

In a preferred embodiment, the aqueous urea solution comprises water and urea in a ratio of 21 to 25% by weight of water to 75 to 79% by weight of urea, based on the total weight of water and urea, more preferably in a ratio of 23.1% by weight of water to 76.9% by weight of urea. The weight ratio of 23.1% by weight of water to 76.9% by weight of urea corresponds to a stoichiometric ratio of water to urea of 1:1, which means that the urea present in the aqueous urea solution reacts completely with the water present therein to give ammonia and carbon dioxide. If the aqueous urea solution comprises less urea, the water is not converted completely; if the aqueous solution comprises more urea, isocyanic acid is also formed as well as ammonia and carbon dioxide. For a maximum yield of ammonia and carbon dioxide, the ratio of water to urea should be within the aforementioned range.

In a further preferred embodiment, the aqueous urea solution comprises 30% by weight to 34% by weight of urea and 70% by weight to 66% by weight of water, based on the total weight of the urea solution.

A urea solution with 32.5% by weight of urea is optimal, since the solution at this concentration forms a eutectic mixture which remains liquid down to −11° C. This simplifies the solution preparation, and the transport and storage of the solution. When the urea concentration is increased, the stability of the urea solution is guaranteed only by the constant supply of heat. This makes the process costly and unprofitable when the outside temperatures are lower.

One use of such a mixture is as a reducing agent for offgas aftertreatment of nitrogen oxide emissions, and is commercially available under the Ad Blue® name. The urea concentration of the aqueous urea solution used as a reducing agent is fixed in ISO 22241-1 at 31.8 to 33.2% by weight of urea, based on the overall solution. The aqueous urea solution used most preferably has a content of 31.8 to 33.2% by weight of urea and 76.8 to 78.2% by weight of water, based on the total weight of water and urea.

In addition, the aqueous urea solution may comprise at least one surface-active component (surfactant), preferably 0.1 to 5% by weight of at least one surfactant, more preferably 0.5 to 1% by weight of at least one surfactant, based on the total weight of the urea solution. The surface-active components used may be anionic, cationic and nonionic surfactants.

Commonly used nonionic surfactants are, for example, ethoxylated mono-, di- and trialkylphenols, ethoxylated fatty alcohols and polyalkylene oxides. In addition to the unmixed polyalkylene oxides, preferably C₂-C₄-alkylene oxides and phenyl-substituted C₂-C₄-alkylene oxides, especially polyethylene oxides, polypropylene oxides and poly(phenylethylene oxides), particularly block copolymers, especially polymers having polypropylene oxide and polyethylene oxide blocks or poly(phenylethylene oxide) and polyethylene oxide blocks, and also random copolymers of these alkylene oxides. Such alkylene oxide block copolymers are known and are commercially available, for example, under the Tetronic and Pluronic names (BASF).

Typical anionic surfactants are, for example, alkali metal and ammonium salts of alkyl sulfates (alkyl radical: C₈-C₁₂), of sulfuric monoesters of ethoxylated alkanols (alkyl radical: C₁₂-C₁₈) and ethoxylated alkylphenols (alkyl radicals: C₄-C₁₂), and of alkylsulfonic acids (alkyl radical: C₁₂-C₁₈).

Suitable cationic surfactants are, for example, the following salts having C₆-C₁₈-alkyl, alkylaryl or heterocyclic radicals: primary, secondary, tertiary or quaternary ammonium salts, pyridinium salts, imidazolinium salts, oxazolinium salts, morpholinium salts, propylium salts, sulfonium salts and phosphonium salts. Examples include dodecylammonium acetate or the corresponding sulfate, disulfates or acetates of the various 2-(N,N,N-trimethylammonium)ethylparaffin esters, N-cetylpyridinium sulfate and N-laurylpyridinium salts, cetyltrimethylammonium bromide and sodium laurylsulfate.

The presence of the surface-active component promotes oil extraction and the formation of foam-like structures. The surface-active component is already present where the gases form in the aqueous solution. In the known processes for foam formation involving serial pumping of water portions with surface-active components and pumping of gas portions, a large portion of the water comprising surface-active substances does not come into contact with the gases underground. The formation of a homogeneous bank of foam-like structures in the productive zone of the mineral oil deposit is barely possible in this case, since the gases escape principally into the upper edge zone of the reservoir of the mineral oil deposit, and the liquid phase (surfactant-containing water) into the zone close to the brine. The mixing of the gases with the liquid in the reservoir zones which are relatively far away from the injection well is barely possible in the case of serial pumping of liquid and gas. As already mentioned above, the addition of surfactants to the aqueous urea solution increases oil extraction and the formation of foam-like structures, especially in combination with the alkali which forms after hydrolysis of the urea in the deposit.

In a further embodiment, the aqueous urea solution comprises at least one ammonium salt, preferably 5 to 30% by weight of at least one ammonium salt.

The ammonium salts may be selected, for example, from ammonium chloride, ammonium bromide, ammonium formate and ammonium nitrate.

The ammonium salts form an alkaline pH buffer system with the ammonia formed in the hydrolysis; this is advantageous especially in the presence of surface-active components since the alkaline pH buffer system ensures the good wash properties of the surfactants.

The use of ammonium formate has the advantage that ammonium formate can likewise decompose to ammonia and formic acid, and the formic acid decomposes further to carbon monoxide and water. For example, it is possible to use a mixture of 20% by weight of urea, 26% by weight of ammonium formate and 44% by weight of water, known commercially as Denoxium-30. This mixture is particularly preferred since it has a comparatively low crystallization point of about −30° C. It is particularly important in development of the deposits in northern regions where the outside temperatures are low.

In a further embodiment of the invention, the aqueous urea solution comprises, as well as urea, at least one surfactant and at least one of the aforementioned ammonium salts. Preference is given to the concentration ranges stated above in each case.

At least one of the above-described aqueous urea solutions is injected into the injection well connected to the mineral oil deposit via a perforation zone, and pressure-injected into the mineral oil deposit through the perforation zone. In the context of the present invention, “aqueous urea solution” is used synonymously with “at least one aqueous urea solution”.

Preferably, the at least one aqueous urea solution is injected into the at least one injection well in serial portions and pressure-injected into the deposit through the at least one perforation zone. This means that the aqueous urea solution is injected in several portions in succession, optionally alternately with other flood solutions or flooding water, and pressure-injected into the deposit. The portion volume typically used for the aqueous urea solution depends on the properties of the deposit and of the mineral oils and may vary, for example, between 100 and 1000 m³. Experience has shown that the amount of urea in one portion should be at least 15 to 20 tonnes, calculated as dry urea. The pumping of such a portion of the aqueous urea solution normally does not take longer than one to three days. This is followed by further flooding with water or steam. The flooding with aqueous urea solution can be repeated cyclically, for example at a time interval of one to three months. It is also possible to heat the flooding water in all flooding phases. In this case, flooding is effected with warm water/warm aqueous solution.

The at least one urea solution has a temperature of at least 80° C. on pressure injection into the deposit.

In general, the temperature that the aqueous urea solution has on pressure injection is selected taking account of the following influencing factors:

-   -   temperature of the rocks in the zone close to the borehole,     -   length of the injection borehole,     -   the injection rates,     -   the temperature gradient of the deposit.

The aqueous urea solution may be provided already with the desired temperature of at least 80° C., for example by using water of appropriate temperature to make up the aqueous urea solution.

The aqueous urea solution can, however, also be provided above ground with a temperature below 80° C. and then heated to at least 80° C. in the injection well. This embodiment is preferred. According to the temperature of the aqueous urea solution provided, there is a greater or lesser temperature difference from the desired temperature on pressure injection, which means that more or less energy has to be supplied for heating.

Preferably in accordance with the invention, warm accompanying water which has also been produced in oil production is used to prepare the aqueous urea solution. This water, however, need not necessarily already have a temperature of at least 80° C. The water preferably has a temperature below 80° C.

In one embodiment of the present invention, the urea solution on pressure injection into the deposit has a temperature of at least 80° C., but below the boiling temperature of the urea solution. The boiling temperature of the urea solution depends on the concentration and the pressure existing in the deposit. The urea hydrolysis is preferably completed here in the region close to the injection well. This variant is used predominantly in development of deposits whose temperatures are below 80° C. However, the process can also be used in development of deposits with temperatures of 80° C. or more. What is meant here in each case is the temperature of the deposit directly before use of the process according to the invention.

In a further embodiment of the present invention, the aqueous urea solution is vaporized in the well prior to pressure injection, and the resulting steam or the gas mixture is pressure-injected into the deposit. This means that the temperature T_(V) is above the boiling temperature of the aqueous urea solution. The vaporization of the aqueous urea solution is preferably performed directly in the injection well. The vaporization more preferably takes place above the perforation zone, and a mixture of steam, carbon dioxide, ammonia and possibly further products of the urea hydrolysis is pressure-injected into the deposit.

The heating of the at least one aqueous urea solution to at least 80° C. can take place above ground using conventional techniques for water heating. The hot solution is injected into the injection borehole and pressure-injected through the perforation zone into the deposit. Since the transport speed of the hot solution in the injection well is relatively high, the aqueous urea solution is cooled only insignificantly during the injection and pressure injection. In order to minimize possible cooling effects, heat-insulating pipes can also be used in the injection well.

The at least one aqueous urea solution can also be heated to at least 80° C. in the injection well. This embodiment is preferred. For this purpose, suitable apparatuses may be arranged in the injection well, preference being given to arranging electrical and/or inductive heating elements above the perforation zone within the injection well, with which the aqueous urea solution is heated in the injection well.

The aqueous urea solution is heated by the electrical and/or inductive heating elements in the injection well to preferably at least 80° C., more preferably to at least 85° C., and very preferably to at least 90° C.

With particular preference the aqueous urea solution is at least partly vaporized in the injection well by the electrical and/or inductive heating elements. With particular preference the aqueous urea solution is vaporized completely.

In one preferred embodiment, the water of the aqueous urea solution may be vaporized, by virtue of the high pressure within the injection well, at temperatures in the range from 200 to 300° C., preferably 200 to 250° C. and more preferably 230 to 250° C. At these temperatures, the urea becomes substantially completely hydrolyzed within a very short time.

The full decomposition of the urea may also, however, take place below the vaporization temperature.

In one preferred embodiment, the at least one aqueous urea solution is heated in the injection well above the perforation zone, before being injected into the mineral oil deposit, to a temperature in the range from 230° C. to 250° C.

In accordance with one preferred embodiment, an aqueous urea solution with a weight ratio of 21 to 25% by weight of water to 75 to 79% by weight of urea, preferably of 23.1% by weight of water to 76.9% by weight of urea, based on the total weight of water and urea, is heated/vaporized by the heating elements in the injection well until the urea has been essentially fully hydrolyzed, and is then pressure-injected through the perforation zone into the mineral oil deposit together with steam or hot water. “Essentially fully hydrolyzed” preferably means that at least 90% of the urea, more preferably at least 95% and very preferably at least 99% of the urea is present in hydrolyzed form, based on the total amount of the urea present in the aqueous solution. For this purpose, the aqueous urea solution can be heated to temperatures of 150 to 300° C. in the injection well, for example. In accordance with this embodiment, the aqueous urea solution preferably comprises essentially water and urea; more preferably, it consists of water and urea.

In the vaporization of aqueous urea solutions of the composition described above, the full hydrolysis of the urea is accompanied by full consumption of the water for the formation of carbon dioxide and ammonia. In this case only gas is pressure-injected into the mineral oil deposit (pure gas flooding).

The heating/vaporization of the aqueous urea solution takes place preferably above the perforation zone in the lower region of the injection well, in other words in the bottom 50%, preferably in the bottom 30%, more preferably in the bottom 20% and more particularly in the bottom 10%, based in each case on the overall length of the injection well.

The overall length of the injection well here means the section (the length) between borehole head and perforation zone. Preferably, therefore, the heating elements as well are arranged above the perforation zone in the lower region of the injection well, in other words in the bottom 50%, preferably in the bottom 30%, more preferably in the bottom 20% and more particularly in the bottom 10%, based in each case on the overall length of the injection well.

The at least one aqueous urea solution can also be heated to at least 80° C. in several steps, the heating comprising the following steps:

-   (a) heating the at least one aqueous urea solution to a temperature     below 80° C., and -   (b) heating the at least one aqueous urea solution to at least 80°     C., preferably at least 85° C. and more preferably at least 90° C.

In a preferred embodiment, the at least one aqueous urea solution is heated in step b) to a temperature in the range from 200 to 300° C., preferably 200 to 250° C. and more preferably 230 to 250° C.

Preferably, step (a) is performed outside the injection well and step (b) within the injection well. Likewise preferably, step (a) is performed at the borehole head of the injection well. More preferably, step (a) is performed at the borehole head of the injection well and step (b) within the injection well. If only the two steps (a) and (b) are performed for heating, this is a two-stage heating operation.

Depending on the temperature that the aqueous urea solution has on pressure injection, the noticeable hydrolysis of urea commences in the first few hours or the first few minutes after attainment of this temperature. This time is normally sufficient to avoid the unwanted commencement of intensive urea hydrolysis within the injection well, since the flow rate of the aqueous urea solution within the injection well is rapid compared to the flow rate in the mineral oil deposit, and the intensive urea hydrolysis therefore commences essentially in the zone close to the borehole, in which the flow rate of the aqueous urea solution declines significantly. This is also the case when the hydrolysis already begins in the injection borehole to such a significant degree that a mixture comprising water, urea and gases already formed is pressure-injected into the deposit. Since the hydrolysis of the urea starts up rapidly after the attainment of at least 80° C., but a few minutes or even hours are required for the completion, the complete hydrolysis of the urea takes place in the zone close to the injection well.

Even in the case that the temperature of the rocks in the zone close to the borehole is much lower than the urea hydrolysis temperature, the urea hydrolysis takes place to a sufficient degree, since comparatively large masses of aqueous urea solution are pressure-injected, and hence the cooling rate thereof is low.

In addition to the decomposition of the urea, the thermal contact can bring about controlled vaporization of the water in the aqueous urea solution. This establishes non-eutectic mixing ratios between water and urea, which leads to a change in the properties of the aqueous urea solution.

In a further embodiment, an aqueous urea solution with a weight ratio of 21 to 25% by weight of water to 75 to 79% by weight of urea, preferably of 23.1% by weight of water to 76.9% by weight of urea, based on the total weight of water and urea, is heated above ground until the urea has been essentially fully hydrolyzed, and then injected into the at least one injection well together with steam or hot water and pressure-injected into the deposit. “Essentially fully hydrolyzed” preferably means that at least 90% of the urea, more preferably at least 95% and most preferably at least 99% of the urea is present in hydrolyzed form. For this purpose, the aqueous urea solution can be heated to temperatures of 150 to 300° C., for example in a vessel directly at the drilling site. In this variant of the process according to the invention, heating elements arranged in the injection well are superfluous. In this embodiment, the aqueous urea solution preferably comprises essentially water and urea; more preferably, it consists of water and urea.

In the case of the vaporization of aqueous urea solutions of the above-described composition, in the complete hydrolysis of the urea, the water is completely consumed for the formation of carbon dioxide and ammonia. In this case, only gas is pressure-injected into the mineral oil deposit (pure gas flooding).

In principle, the higher the urea concentration of the aqueous urea solution, the less additional water, which is not required for the hydrolysis of urea, need also be heated; this saves costs since the volume of the aqueous urea solution is reduced. However, the urea concentration is limited by the possible crystallization of the urea solution and depends on the temperature.

In a further advantageous embodiment of the present invention, the process is performed by pressure-injecting at least one aqueous urea solution comprising only water and urea as components, after heating to the temperature T_(V), into the deposit, it being preferable that at least the last stage of the heating in the injection tube is effected by means of a heater arranged therein. Subsequently, with the heater switched off in the injection tube, one or more aqueous solutions comprising at least one additive selected from surface-active components and ammonium salts can be pressure-injected into the deposit. In this case, the aqueous additive solution mixes with the hydrolyzing urea solution directly in the deposit. The serial pumping of aqueous urea solution comprising only water and urea, and aqueous solution comprising additives, saves costs and, in cases where additives are thermally sensitive, protects the additive solution. The sequence of pumping of the individual solutions may be different, for example

-   -   additive solution (heater passive)→urea solution (heater         active)→additive solution (heater passive) or     -   additive solution (heater passive)→urea solution (heater active)         or     -   urea solution (heater active)→additive solution (heater passive)         or     -   urea solution (heater active)→additive solution (heater         passive)→urea solution (heater active).

“Heater active” means that the heater is heating; “heater passive” means that it is not heating.

Between the individual portions, buffer water can also be pressure-injected. The possible sequences shown above can be repeated cyclically.

FIG. 1 shows a typical injection well, as usable in the process according to the invention. It comprises a well with an injection tube (5) enclosed by the feed tube/casing (14 a), the injection tube/riser tube (5) being sealed from the well by what is called a packer (13). In the perforation zone (4) the feed tube (14 a) has perforation holes (6) through which the urea solution (7) injected into the injection tube is pressure-injected into the deposit (2).

FIGS. 2 and 3 show two injection wells which can be used for preferred embodiments of the present process.

In FIG. 2, two heaters/heating elements (3) are arranged around the injection tube (5). In the present context, the terms “heaters” and “heating elements” are used synonymously. In addition, the borehole head (8), the vessel (9) for the flood solution/urea solution and the pump (12) are shown.

FIG. 3 shows an injection well in which the aqueous urea solution is heated in two stages. For instance, the vessel for the flowing solution/aqueous urea solution (9) may comprise a heating element (11). The injection well head (8) may likewise comprise a heating element (10). In FIG. 3, a heating element (3) is arranged on the injection tube (5).

In a further embodiment, after the completion of the pressure injection of the urea solution, the heaters remain switched on and the flooding water pressure-injected subsequently is brought to at least 80° C., preferably to at least the temperature that the aqueous urea solution had on pressure injection. This prevents the rapid cooling of the heated urea solution by mixing with cold water, and enables very substantially complete urea hydrolysis. The volume of the subsequently pressure-injected flooding water with this temperature may be equal to the volume of the pressure-injected urea solution.

The same aim is pursued by a further embodiment, in which, before and after the at least one aqueous urea solution, at least one portion of flooding water is injected through the at least one injection well and pressure-injected into the deposit, the flooding water on pressure injection having a temperature of at least 80° C. in each case. In this embodiment, two heat buffers are formed. The use of this embodiment is appropriate particularly in development of deposits with temperatures of 8 to 40° C.

The heating element(s) (3) arranged in the injection tube can be used to heat the aqueous urea solution to a temperature above the hydrolysis temperature of the urea, the temperature being below the boiling temperature of the aqueous urea solution, but the aqueous urea solution can also be heated in the injection tube to such an extent that it vaporizes in the injection well. The person skilled in the art is aware of different heaters for heating and/or vaporization of flooding water, which are installed in the borehole itself; see, for example, RU 2086759, RU 2198284, U.S. Pat. No. 5,465,789 and U.S. Pat. No. 5,323,855. It is possible in principle to mount one heating element, or else two or more heating elements, along the injection tube.

For example, an inductive heating element (3), which is configured, for example, as a coil with housing, may be mounted on the riser tube/injection tube (5), and the riser tube/injection tube (5) can be used as a ferromagnetic core. The heater may also be an electrical heater. A heater typically comprises one or more heating elements. In the process according to the invention, on commencement of flooding with aqueous urea solution, the heater (3) which heats the injection tube (5) and the aqueous urea solution flowing through is switched on.

The aqueous urea solution heated to at least 80° C. is pressure-injected through the perforation orifices (6) into the deposit. This aqueous urea solution comprises, water, urea, and hydrolysis products of the urea hydrolysis which has already at least partly taken place, comprising ammonia and carbon dioxide, and possibly further additives present, such as surfactants and ammonium salts. If the aqueous urea solution has a temperature below the boiling temperature of the aqueous urea solution, the urea hydrolysis in this embodiment is preferably completed in the region close to the injection well. This variant is used predominantly in development of deposits with temperatures below 80° C. The process can, however, also be used in development of deposits with temperatures above 80° C. After the flooding phase with aqueous urea solution has ended, the heater (3) is switched off and the deposit is flooded with water. The pumping of the aqueous urea solution can be repeated cyclically. It is also possible to heat the flooding water in all flooding phases. In this case, flooding is effected with warm water/warm aqueous solution.

The aqueous urea solution and the flooding water are pumped, for example, with a conventional pump (12) situated above ground.

The structure shown in FIG. 3 is suitable, for example, for the inventive multistage heating of the aqueous urea solution. In this embodiment of the invention, the heating comprises the following two steps:

-   a) heating the aqueous urea solution to a temperature below 80° C.,     and -   b) heating the aqueous urea solution to at least 80° C., preferably     at least 85° C. and more preferably at least 90° C.

Preferably, step a) is performed outside the injection well, for example in the tank (9) or the borehole head of the injection well, and step b) within the injection well. For this purpose, additional heaters (10) and (11) are installed in the injection well head (8) or in the vessel (9) for the aqueous solution. The second heating stage is preferably executed with the aid of the heater (3) directly in the injection well before the solution is pressure-injected into the deposit (2). This embodiment has the advantage that the heater (3) can be operated underground with minimal electrical power since the necessary temperature rise (T_(d)) of the aqueous solution to the desired temperature on pressure injection into the deposit can be kept as small as possible. This simplifies the application and increases the reliability of the apparatus, since only a small amount of corrosion-hazardous carbon dioxide, if any, is generated during the transport of the urea solution within the riser tube/injection tube (5). Carbon dioxide occurs to a significant degree in the zone close to the borehole only. Thus, the corrosion risk for the borehole equipment is minimal. When the urea solution is heated above ground (for example vessel (9) or at the borehole head (8)), the riser tube/injection tube (5) can be provided with heat insulation.

If the power of the heater (3) is insufficient to rapidly heat up the desired amount of aqueous urea solution, the flood rates in the flooding with the aqueous urea solution are reduced.

In order to enhance the efficiency of the heaters (3), in the case of employment of induction heaters, spiral-like ribs can be installed in a fixed manner in the injection line. In this case, the ribs and the corresponding sections of the injection tube (5) are heated inductively to temperatures of 100 to 500° C. This allows the enhancement of the flow turbulences in the heated section of the injection tube (5) and rapid transfer of the thermal energy to the aqueous urea solution. In the case of an abrupt rise in the temperature of the aqueous urea solution from a temperature below the hydrolysis temperature to a temperature above the hydrolysis temperature of the urea, it is possible in this way to induce the commencement of the urea hydrolysis.

FIGS. 4 a and 4 b show the temperature profile of the aqueous urea solution along the injection well from the borehole head to the perforation zone (4). The arrow labeled T_(L) represents the temperature of the solution in the manner of an abscissa, with a dotted line drawn in for the temperature 80° C. in each case. The arrow labeled “d” vertically downward represents a measure of the depth of the well. T_(d) denotes the temperature jump in the aqueous urea solution caused by heater (3).

In FIG. 4 a, the aqueous urea solution has not been heated beforehand, but rather is injected directly into the injection well, the ambient temperature in the surrounding rock being higher, such that the aqueous urea solution heats up gently until it reaches the heater (3). According to FIG. 4 b, the aqueous urea solution was heated to a temperature which is below the hydrolysis temperature of the urea but is much higher than the temperature according to 4 a. In addition, either the injection tube was provided with thermal insulation, or else the surrounding rock has a similar temperature to the urea solution injected, such that the temperature of the aqueous urea solution is unchanged from the injection borehole head until it reaches the heater (3). It becomes clear from the temperature profiles that the heater (3) in the process shown in FIG. 4 a must bring about a much greater temperature rise than in the process according to FIG. 4 b.

The invention has the following advantages:

-   -   simple design,     -   controlled and complete hydrolysis of the urea,     -   no losses of the urea in the deposit,     -   no deposit temperature limit for the use of the process,     -   increase in oil extraction.

LIST OF FIGURES

FIG. 1: Vertical section of an injection well in the lower region in the injection well

FIG. 2: Vertical section of the injection well in the case of one-stage heating of the flood solution

FIG. 3: Vertical section of the injection well in the case of two-stage heating of the flood solution

FIG. 4 a: Temperature profile of the flood solution in the injection well in a one-stage heating process

FIG. 4 b: Temperature profile of the flood solution in the injection well in a two-stage heating process

LIST OF REFERENCE NUMERALS USED

-   1: Injection well -   2: Deposit/reservoir/carrier -   3: Heating elements (electrical or inductive) -   4: Perforation zone -   5: Riser tube/injection tube -   6: Perforation orifices -   7: Aqueous mixture comprising urea -   8: Injection well head -   9: Vessel -   10: Heater at the borehole head -   11: Heater in the vessel -   12: Pump -   13: Packer -   14: Thermally resistant surround (for example cementation) -   14 a: Feed tube/casing 

1. A process for producing mineral oil from an underground mineral oil deposit into which at least one injection well which is in contact with the mineral oil deposit via at least one perforation zone and at least one production well have been sunk, comprising injecting at least one aqueous urea solution into the injection well and pressure-injecting the aqueous urea solution through the perforation zone into the mineral oil deposit, wherein the aqueous urea solution on pressure injection into the mineral oil deposit is at a temperature of at least 80° C., wherein the at least one aqueous urea solution is heated to at least 80° C. in the injection well by means of electrical and/or inductive heating elements arranged above the perforation zone in the injection well.
 2. The process according to claim 1, wherein the at least one aqueous urea solution is injected into the at least one injection well in serial portions and pressure-injected into the deposit through the at least one perforation zone.
 3. The process according to claim 1, wherein the at least one aqueous urea solution is heated to at least 80° C. in two or more stages, the heating comprising the following stages: (a) heating to a temperature below 80° C., and (b) heating to at least 80° C.
 4. The process according to claim 3, wherein stage (a) is performed outside the injection well and stage (b) within the injection well.
 5. The process according to claim 1, wherein the at least one urea solution on pressure injection into the mineral oil deposit is at a temperature below the boiling temperature of the at least one urea solution.
 6. The process according to claim 1, wherein the at least one aqueous urea solution is vaporized in the injection well prior to pressure injection into the deposit.
 7. The process according to claim 1, wherein, before and after the at least one aqueous urea solution, at least one portion of flooding water is injected through the at least one injection well and pressure-injected into the deposit, the flooding water on pressure injection having a temperature of at least 80° C. in each case.
 8. The process according to claim 1, wherein the at least one aqueous urea solution comprises 3 to 79% by weight of urea and 21 to 97% by weight of water, based on the total weight of the urea solution.
 9. The process according to claim 1, wherein the at least one aqueous urea solution comprises 0.1 to 5% by weight of at least one surface-active component, based on the total weight of the urea solution.
 10. The process according to claim 1, wherein the at least one aqueous urea solution comprises 5 to 30% by weight of at least one ammonium salt, based on the total weight of the urea solution.
 11. The process according to claim 1, wherein the ratio of water to urea in the at least one aqueous urea solution is 21 to 25% by weight of water to 75 to 79% by weight of urea, based on the total weight of water and urea.
 12. The process according to claim 1, wherein, alternately with the at least one aqueous urea solution, at least one further aqueous flood solution comprising at least one additive selected from surface-active components and ammonium salts is injected into the at least one injection well and pressure-injected into the deposit.
 13. The process according to claim 12, wherein there is no heating in the injection well on injection of the at least one further flood solution comprising at least one additive selected from surface-active components and ammonium salts into the injection well and pressure injection into the deposit.
 14. The process according to claim 1, wherein the heating elements are arranged above the perforation zone in the lower region of the injection well.
 15. The process according to claim 1, wherein the aqueous urea solution is heated or vaporized by the heating elements until at least 90% of the urea is present in hydrolyzed form. 